Char the onion and ginger over an open flame or under a broiler until blackened and fragrant.
In a large pot, toast the star anise and cinnamon stick for 1-2 minutes until aromatic.
Add the beef bone broth, charred onion, ginger, and fish sauce to the pot and simmer for 20 minutes.
While the broth simmers, cook the rice noodles according to package directions and drain.
Thinly slice the raw flank steak against the grain into very thin, translucent pieces.
Strain the broth to remove the solids and return the clear liquid to the pot, bringing it to a rolling boil.
Place the cooked noodles in a large bowl and top with the raw beef slices.
Ladle the boiling broth over the beef to cook it instantly, then garnish with bean sprouts, basil, jalapeño, and lime.